Hello,
Actually there is no standard size when you change the header with your image, but by default machform form width is 640px. it means you can use that width as your default, otherwise you need to adjust your form width also.
To change the header, you can search logo section in your "CSS FORM", you'll see this code :
#main_body h1
{
background-color:#dedede;
margin:0;
min-height:0;
padding:0;
text-decoration:none;
text-indent:-8000px;
background-image: url('../../../images/machform.gif');
background-repeat: no-repeat;
}
Change background-image value with your header image path, for example :
background-image: url('http://www.yourdomain.com/images/header.jpg');
Anyway I'm not sure with your second question, do you wan't to add some menu bellow form header ? If it's so you need to edit "includes/view-function.php" file, go to around line 1839 and you'll see this code :
<h1><a>{$form->name}</a></h1>
add "div" line along with menu/navigation script, bellow that code. For example :
<div>
<!--your menu here -->
</div>